1. Field of the Invention
The present invention generally relates to a wireless communication system. More particularly, the present invention relates to a method and apparatus for normalizing input metric to a channel decoder.
2. Description of the Related Art
Code division multiple access 2000 (CDMA 2000), wideband-CDMA (WCDMA) and institute of electrical and electronics engineers (IEEE) 802.16 systems perform modulations of quadrature phase shift keying (QPSK), 8PSK, 16-ary quadrature amplitude modulation (16-QAM), 64-ary quadrature amplitude modulation (64-QAM) and so on. Further, these systems perform adaptive modulation and coding (AMC) with a combination of channel codes such as turbo codes. The systems obtain an optimal transmission rate proper for a channel situation. A reception stage computes a log likelihood ratio (LLR) per bit with a demapper according to various modulations and acquires input metric to a channel decoder. The channel decoder receives and decodes the metric.
FIG. 1 illustrates a structure of a transceiver in a conventional wireless communication system.
Referring to FIG. 1, binary data i(n) to be transmitted is encoded in a channel encoder 110 within a transmitter 100. The channel encoder 110 generates a series of binary code symbols c(n). A mapper 120 generates a block of several code symbols of the generated code symbols, performs mapping to one point on a signal constellation, and performs transformation into a modulation symbol x(n) of a complex value. The modulation symbol x(n) is applied to a modulator 130. The modulator 130 generates a continuous-time wave in a code division access multiplexing (CDMA) or orthogonal frequency division multiplexing (OFDM) scheme according to modulation symbol x(n) and transmits the generated wave to a receiver 150 through a channel 140.
In the receiver 150, a demodulator/channel estimator 160 performs baseband demodulation and channel estimation processes for a received signal: The demodulator can be implemented according to various technologies. For example, the demodulator can be an OFDM demodulator implemented with a CDMA Rake receiver or an inverse fast Fourier transform (IFFT) processor and a channel estimator. After the baseband demodulation, a channel estimate c(n) and a received symbol y(n) modulated by QAM or PSK are obtained.
A demapper 170 computes metric of bits constructing a codeword of channel codes using the received symbol y(n) and the channel estimate c(n). A sequence Λ(n) corresponding to a metric value computed in the demapper 170 is input to a channel decoder 180 and is decoded into originally transmitted binary data. When the channel decoder 180 completes the decoding operation, the receiver 150 completes a basic operation in a physical layer. At this time, the channel decoder 180 may use a Viterbi decoder for convolutional codes, a soft output Viterbi algorithm (SOVA) iterative decoder for turbo codes, a log-maximum a posteriori (MAP) iterative decoder, and a max-log-MAP iterative decoder, and so on.
In the implementation of the conventional wireless communication system operating as described above, a dynamic range of metric input to the decoder is not limited when a floating-point operation is conventionally performed. However, when hardware for performing a fixed-point operation is implemented, it is affected by quantization noise, clipping noise, and so on according to dynamic range. Therefore, each step of a communication system should ensure optimal performance with minimal hardware by performing normalization proper for metric representation. However, since the conventional method does not consider normalization of metric computed in a demapper, there is a problem in that the performances of a high code rate and high-order modulation are lower than those of the conventional code rate and modulation.